Driving motors have been used as a prime mover not only in an electric vehicle but also in a hybrid vehicle using both an internal combustion engine and an electric motor. In the electric vehicle, the driving motors are arranged to drive each front wheel or each rear wheel. Alternatively, the driving motors may also be arranged to drive all wheels. Thus, in those kinds of vehicles, the driving motors are provided to drive the right wheel and the left wheel individually. Therefore, if one of those driving motors is in trouble, or if one of electric current sources is in trouble, the driving force of the right and the left wheels lose their balance.
Japanese Patent Laid-Open No. 4-145810 discloses an electric motor vehicle comprising electric motors for driving right and left wheels, and inverters for feeding electric current to those motors. According to the teachings of Japanese Patent Laid-Open No. 4-145810, in case a failure occurs in one of the inverters, a current supply from the troubled inverter is interrupted, and the other inverter supplies the current to both right and left wheels to drive those wheels. According to another embodiment of Japanese Patent Laid-Open No. 4-145810, a clutch is individually interposed between each motor and wheel. According to this embodiment, in case a failure occurs in one of the inverters, a current supply from the troubled inverter is interrupted while disengaging the clutch attached to the motor connected with the troubled inverter.
Japanese Patent Laid-Open No. 2009-142036 discloses another example of the electric vehicle having motors for driving each of the front and rear wheels. In order to reduce drag loss of the motors to be rotated in case of driving only one of the pairs of front and rear wheels, according to the electric vehicle taught by Japanese Patent Laid-Open No. 2009-142036, a one-way clutch adapted to transmit the power only in one rotational direction is individually interposed between each motor and wheel.
According to the electric motor vehicle taught by Japanese Patent Laid-Open No. 4-145810, even in case a failure occurs in one of the inverters for feeding the current to the driving motor, both of the right and the left motors can be driven by supplying the current thereto from the other inverter while stopping a current supply from the troubled inverter. Under the normal condition, those motors are driven individually by the currents of different phases outputted from different inverters. However, provided that both of the right and the left motors are synchronous motors, those motors may be driven out of phase if those motors are thus driven individually by the currents of different phases. Therefore, if those motors thus rotated out of phase are driven together by the common current outputted from the one of the inverters when the other inverter is in trouble, a torque balance between the right and the left wheel may be lost.